The result is admittedly close to perfection: a series of smoky blue-black floor-to-ceiling glass panels with alternating matt and polished finishes that create a striking sense of simple sophistication. The 84 studio apartments upstairs also sport Citterio's trademark modernist style, with a subtle cool silver-grey palette and pared-back décor.

His previous projects include designing über-chic hotels for Bulgari - converting a former convent into the Bulgari Hotel in Milan in 2004, followed by the Bulgari Resort in Bali and the Bulgari restaurant and cafe in Tokyo, in 2007. He is also responsible for the sleek interiors at the W hotel in St Petersburg.

Still, when he first saw 99 Bonham Strand, Citterio says he was "completely seduced by the pictures of the site".

He had just completed another project with the exact same proportions, only where that was horizontal, 99 Bonham was vertical. He said that aspect plus the "intense" urban context intrigued him.

"Hong Kong is one of the top cities in the world," he says, waving enthusiastically at the view towards the Cesar Pelli-designed IFC in Central.

"When I went to New York for the first time in the 1970s it was the centre of the world for a young guy. I feel that same feeling now about Hong Kong. There is an energy here that you don't have elsewhere."

We've met in one of 99 Bonham's largest studio apartments, which, at 600 square feet, appears curiously more spacious than it should, thanks to its carefully executed proportions, subtle arrangement of space and custom-made furniture. Citterio's designs - much like the person himself - are never obvious, tending towards quiet efficiency rather than showy ostentation.

He explains that creating defined areas within one integrated space was central to his design concept. Near the door is a compact but beautifully designed bathroom/dressing room complete with Citterio-designed Hansgrohe taps and the Mobil drawer system he designed for Kartell in 1994 (now part of the New York Museum of Modern Art's permanent collection). A king-size Simmons bed dressed in Frette bed linen takes pride of place, and near the window is a glass table large enough to seat five people comfortably. "The studio's area is limited but you don't feel like you're in a small room," he says. "The room has a precise sequence that precisely addresses moving from privacy to maximum visibility at the window. The space challenge makes you work hard on the furniture and details - but then we like to solve problems."

The designer admits that he relished the opportunity to create a space that reflected his personal style rather than a corporate brand. "There is a niche in the market [for people] who like contemporary and modesty, like me.

"[The interior] has a calm quality and is something you appreciate after some time. We wanted to create a sense of being in a private apartment," he says, pointing at a luxuriously thick rug and the aluminium Kelvin LED table lamp he designed for Flos.

"What you see here is pretty close to a suite, but it is not. It is a residential product with the feeling of a luxury suite. All the details are important; even the rug is not supposed to feel like a hotel rug. Nothing is done for nothing. Everything has a reason or is intentional."

Viel nods vigorously, saying: "We made many prototypes … many versions and sizes of the furniture to achieve this comfortable feeling of being at home."

Achieving a balance of form, function and craftsmanship has clearly always fascinated Citterio. After graduating with a degree in architecture from the Polytechnic University in Milan, he initially concentrated more on product design, building an impressive portfolio of loyal clients including Ansorg lighting, Arclinea kitchens, B&B Italia and Flexform furniture firms, and Fusital handle makers.

They were drawn to his elegant, functional designs. Among his best known products are the Ray sofa system and Beverly folding armchair for B&B Italia; and the incredibly light and flexible modular office seating he designed with Glen Oliver Low for Vitra.

Citterio's creative flair extends across a wide spectrum - he designed strikingly modern fitness equipment for Technogym. More recently, an armchair designed for Hermes in 2011 shows his love of updating timeless classics, this time with a remarkable combination of stainless steel and turtledove bull calfskin.

Despite his influence on the world of design (he was once described by MOMA's Paola Antonelli as "an important part of design history"), the 61-year-old is adamant that he is an architect first, product designer second. "It is important to recognise the difference. Designers concentrate on the details but architects must focus on the space."

Now head of Antonio Citterio and Partners, a Milan-based partnership (that he co-founded with Viel in 1999), he has more than 62 staff involved in projects of widely varying scales and covering areas from architecture to industrial design and graphics.

He undertakes architectural projects ranging from residential and commercial complexes to industrial facilities, restructuring and conservation of public buildings as well as planning offices, showrooms and hotels.

Of particular note, in 2002 Citterio designed the Research and Development Centre for B&B Italia at Como in Italy; followed by co-ordination of an image project for De Beers' London and Tokyo jewellery stores.

Important projects in his home country include the restructuring of the historic palazzo comunale of Clusone in Bergamo (2005); Zegna's menswear headquarters in Milan (2008) and the Technogym Village in Cesena (2012).

Since 2007, Citterio has been a professor of design at the Architecture Academy of Mendrisio of the University of Lugano. He received the Royal Designer for Industry award from the Royal Society for the Encouragement of Arts, Manufactures & Commerce in London in the same year.

Earlier this year, the Bulgari Hotel in London was inaugurated, and Citterio's studio is also working on a project for the Havenkwartier in Hasselt, Belgium, in collaboration with local architects De Gregorio & Partners.

As the interview draws to a close, another assistant arrives - this time with coffee in plastic cups with straws - which Citterio accepts with gracious good humour and a polite smile. I leave with the distinct impression that he is already mentally designing a more beautiful version.